This invention relates generally to lime slurries, and more particularly to micro-particulated hydrated lime slurries.
Lime in the form of calcium hydroxide (Ca(OH)2) is used extensively to treat water in potable water treatment plants and industrial wastewater and sludge plants. The lime is typically introduced into the process as a slurry, a paste, or as lime milk. Using the lime in a dry form usually creates dust problems that have to be overcome. To keep the processing costs low, lime slurries are typically prepared at the plant where they are to be used. This eliminates having to pay shipping costs for water which make up between 60% to 98% by weight of the lime slurry
It is thought that the smaller the particle size of the lime in the slurry, the more reactive the lime slurry. It follows that the more reactive the lime, the less lime needed in a water treatment process, and consequently, the treatment plant can be smaller.
Lime is a generic term that is know in the art to mean either quicklime, CaO, or hydrated lime, Ca(OH)2. Quicklime can be converted to Ca(OH)2 by a slaking process where water and CaO are mixed under agitation and temperature to produce Ca(OH)2 known as slaked lime. It is thought that slaked lime slurries contain smaller, more reactive Ca(OH)2 particles than a lime slurry made by mixing water and hydrated lime. The theory is that the hydrated lime particles agglomerate thereby producing a lime slurry containing Ca(OH)2 particles having less effective surface area than a slurry of slaked lime.
The size and quality of slaked lime particles in a slurry are dependent on a number of variables. Particularly, the reactivity, particle size, and gradation of the quicklime used influences the speed of slaking and the temperature of the slaking process. Other variables include the amount of water used, and the quality of the water, for example, the amount and type of water impurities, i.e., sulfites and sulfates. Further, the temperature of the water and the amount of agitation can affect slaked lime quality and particle size. To produce a consistent slaked lime requires controlling numerous process variable which can add to the cost of a water treatment plant.
Slurries made from hydrated lime do not require attention to as many process variable and are easier to produce than slaked lime slurries. To address the larger particle size through agglomeration of hydrated lime particles, it has been found that conditioning the hydrated lime slurry under agitation for 4 to 24 hours permits the agglomerates to break down to individual hydrated lime particles. However, this conditioning step is costly because of the extended processing time needed.
It would be desirable to provide hydrated lime slurries that includes hydrated lime particles as small as the slaked lime particles in slaked lime slurries and which do not require a 4 to 24 hour conditioning time.
In an exemplary embodiment, a method of manufacturing micro-particulated hydrated lime slurries includes the steps of blending dry hydrated lime and water to form a mixture of at least 30 percent weight solids, and mixing the hydrated lime and water mixture under high shear to form a slurry. The method further includes transferring the hydrated lime and water slurry to a storage container, and adjusting the percent weight solids of the hydrated lime and water slurry to a predetermined weight solids content.
Mixing the hydrated lime and water mixture under high shear can include mixing the hydrated lime and water mixture with a high shear disperser and/or with a high shear rotor/stator mixer. The resultant slurry includes hydrated lime particles with a median particle size of about 6 micrometers or less.
An apparatus for manufacturing micro-particulated hydrated lime slurries includes a mixing container having a hydrated lime input port, a water input port, a high shear mixer, and a slurry output port. The high shear mixer includes a motor and a high shear blade or a high shear rotor/stator mixer head operatively coupled to the motor. The mixing container is mounted on at least one load cell to operatively determining the weight of the mixing container including any contents therein.
The apparatus further includes a storage tank having a mixing blade operatively coupled to a motor, a water inlet, a slurry inlet coupled to the mixing container slurry outlet by a slurry transfer pipe, and a slurry outlet. The slurry transfer pipe includes a transfer pump.
The above described method produces hydrated lime slurries that include hydrated lime particles that are as small as the slaked lime particles in a slaked lime slurry. Further, the hydrated lime slurries produced with the above described method have lower grit levels, are less abrasive, have a higher rate of solution, have a better consistency, are free of contaminates, and reduces scaling.